Recovery of the marine bottom environment of a Japanese bay

Abstract

Dokai Bay lies adjacent to Kitakyushu city, one of Japan's major cities with a population of more than 1 000 000. In this city, various heavy chemical-industrial plants have been established since the 1900s. Waste water from the factories and untreated sewage effluent from the city have heavily polluted the water and marine bottom environment of the bay. In the 1960s, this bay was called the ‘dead sea’ owing to the apparent absence of aquatic organisms. Since 1970, environmental recovery projects have been carried out by the local government. The effluent and waste water have been severely regulated. The polluted bottom sediments of the bay have been dredged. Consequently, in the past two decades, the environmental conditions of the bay have dramatically recovered. Since 1989, we have assessed the water and benthic conditions of the bay to describe the recovery of the benthic ecosystems, and to monitor the effects of environmental recovery projects on the bottom environment of the bay since 1970. The results of these studies indicate a drastic decrease in the levels of heavy metals in the bottom sediments and the recolonization of various benthic organisms, although the innermost areas of the bay remain seriously organically polluted. In these areas, in the summer, the benthic communities are seriously damaged owing to the occurrence of anoxic water masses. Prior to 1993, there was no regulation of the amount of nutrient salt loading present in factory waste water and city sewage. Overloading of nutrient salts generates large amounts of primary production by phytoplankton, and results in the overloading of organic matter at the bottom environment in the innermost areas of the bay. For further recovery of the ecosystem it is necessary to control the total amount of loading of nutrient salts to the bay system.